KR101543837B1 - High yield ratio high-strength hot rolled steel sheet having excellent impact resistance and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a high strength, high strength, high strength hot rolled steel sheet excellent in impact resistance and a method of manufacturing the same.
An embodiment of the present invention relates to a steel sheet comprising, by weight%, 0.03 to 0.1% of C, 0.01 to 0.2% of Si, 0.8 to 2.0% of Mn, 0.03 to 0.1% of Sol.Al, 0.005 to 0.3% 0.001 to 0.25% in total of at least one selected from the group consisting of Ti, Nb and V, 0.005 to 0.01% of P, 0.005 to 0.05% of S, 0.0005 to 0.05% of S and 0.001 to 0.01% of N , The balance Fe and unavoidable impurities,
A high strength, high strength, high strength hot rolled steel sheet excellent in impact resistance characteristics, wherein the number of Ti, Nb and V alone or composite carbonitrides having a diameter of 100 nm or more is 5 x 10 ⁸ / cm ² or less and the yield ratio is 0.85 to 0.95.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength and high strength hot rolled steel sheet having excellent impact resistance,

The present invention relates to a method of manufacturing a high-strength hot-rolled steel sheet, which is mainly used for components requiring high strength and excellent fatigue characteristics, such as automobile wheels and members of chassis components. More particularly, Strength hot-rolled steel sheet excellent in impact resistance and excellent in impact resistance characteristics.

In order to obtain high strength, it is usually made by adding C, Si, Mn, Ti, Nb, Mo, V, etc. using high purity steel with minimized impurities in steel.

A description of the known hot-rolled steel sheet manufacturing technology developed to develop such a high-strength hot-rolled steel sheet will be briefly described below.

Patent Documents 1 and 2 of the prior art disclose a technique of adding a small amount of Ti and securing strength by using Mn or the like, and Patent Document 3 discloses a technique of adding a trace amount of Ti, Nb and W, And Patent Document 4 relates to a manufacturing technique of a hot-rolled steel sheet in which Ti and Mo are added and precipitation strengthening of these elements is utilized. Patent Document 5 is a technique of securing strength by utilizing Ti, Nb, Mo, or the like. In addition, Patent Documents 6 and 7 relate to a technique for forming micro-crystal grains by roughly pressing Ti and Nb-added steels in a non-recrystallized region as one of the methods for producing precipitation-strengthening hot-rolled steel sheets.

However, the above-described conventional techniques have problems. In order to secure the yield strength and the tensile strength at the same time, the high-strength hot-rolled steel sheet manufacturing technology which has most of the high-strength composite materials is mainly composed of the employment reinforcement mechanism utilizing Ti, Nb , V, W, etc. These alloying elements are effective for improving the strength of the hot-rolled steel sheet, but they deteriorate the ductility to lower the workability and cause severe segregation in the slab after casting Cracks or defects are formed during molding to deteriorate endothelial property and impact resistance. Further, when the alloy component is excessively added or the hot rolling condition is inadequate, the deformation resistance is drastically increased during the hot rolling due to the occurrence of the dynamic deformation organic precipitation, so that the quality of the rolled plate becomes poor and the precipitation strengthening effect is decreased, There is a problem that it can not obtain.

Japanese Laid-Open Patent Publication No. 1997-209076 Korean Patent Publication No. 10-2005-0113247 Japanese Patent Application Laid-Open No. 2001-334466 Japanese Laid-Open Patent Publication No. 2001-333376 Japanese Patent Application Laid-Open No. 2006-274317 Japanese Patent Application Laid-Open No. 143570/1990 U.S. Patent No. 6290784

One aspect of the present invention is to provide a high strength, high strength, high strength hot rolled steel sheet excellent in impact resistance by controlling the composition and manufacturing method of the steel sheet to suppress formation of coarse carbonitride and controlling the microstructure of the steel sheet and a method of manufacturing the same .

The present invention relates to a ferritic stainless steel comprising, by weight%, 0.03 to 0.1% of C, 0.01 to 0.2% of Si, 0.8 to 2.0% of Mn, 0.03 to 0.1% of Sol.Al, 0.005 to 0.3% 0.001 to 0.25% in total of at least one selected from the group consisting of Ti, Nb and V, 0.001 to 0.25% of P, 0.0005 to 0.05% of S and 0.001 to 0.01% of N, Includes unavoidable impurities,

A high strength, high strength, high strength hot rolled steel sheet excellent in impact resistance characteristics, wherein the number of Ti, Nb and V alone or composite carbonitrides having a diameter of 100 nm or more is 5 x 10 ⁸ / cm ² or less and a yield ratio is 0.85 to 0.95.

The present invention also relates to a ferritic stainless steel comprising, by weight, 0.03 to 0.1% of C, 0.01 to 0.2% of Si, 0.8 to 2.0% of Mn, 0.03 to 0.1% of Sol.Al, 0.005 to 0.3% 0.01 to 0.25% in total of at least one selected from the group consisting of Ti, Nb and V, 0.001 to 0.01% of P, 0.005 to 0.05% of S, 0.0005 to 0.05% of S and 0.001 to 0.01% of N, Continuously casting molten steel containing Fe and unavoidable impurities to obtain a slab;

Cooling the slab so as to satisfy the condition of the following expression (1);

Reheating the cooled slab to 1200 to 1300 占 폚;

Hot-rolling the reheated slab to obtain a hot-rolled steel sheet;

Cooling the hot-rolled steel sheet at a cooling rate of from 10 to 100 DEG C / s from 550 to 750 DEG C; And

Winding the cooled hot-rolled steel sheet at 550 to 750 ° C,

The step of obtaining the hot-rolled steel sheet includes a step of subjecting to hot-rolling the finish to satisfy the following condition (2)

And the yield ratio of the wound hot rolled steel sheet is 0.85 to 0.95. The present invention also provides a method of manufacturing a high strength and high strength hot rolled steel sheet having excellent impact resistance.

 [Relation 1]

Cr (° C./sec) ≥45.5-56.1 [C] +2.1 [Si] -19.2 [Mn] -8.9 [Cr] + 8.0 [Sol.Al]

(Wherein CR represents the cooling rate, and [C], [Si], [Mn], [Cr], [Sol.Al] and [Mo]

 [Relation 2]

(FET-FDT) (占 폚)? 166 - 456 [C] - 27.9 [Mn] + 4.39 [Si] - 28.5 [Mo] - 28.2 [Ti]

(C), [Mn], [Si], [Mo], [Ti] and [Nb] of the element are the same as those of the element Content (% by weight)

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS The various features and advantages and effects of the present invention will become more fully understood with reference to the following specific embodiments.

According to the present invention, it is possible to provide a high strength and high strength hot rolled steel sheet having an impact energy at -20 캜 of 60 J or more and a yield ratio of 0.85 to 0.95 and excellent in impact resistance.

1 is a graph showing the yield ratio (YR = YS / TS) and the impact energy value of the inventive steel and the comparative steel according to an embodiment of the present invention.

Hereinafter, a high strength and high strength steel sheet having excellent impact resistance characteristics, which is one aspect of the present invention, will be described in detail.

In one aspect of the present invention, a high-strength hot-rolled steel sheet excellent in impact resistance and moldability comprises 0.03 to 0.1% of C, 0.01 to 0.2% of Si, 0.8 to 2.0% of Mn, 0.03 to 0.1% of Sol.Al, , 0.005 to 0.3% of Cr, 0.005 to 0.01% of Mo, 0.005 to 0.05% of P, 0.0005 to 0.05% of S and 0.001 to 0.01% of N, 0.001 to 0.25% in total, the balance Fe and unavoidable impurities,

The number of Ti, Nb and V alone or complex carbonitrides having a diameter of 100 nm or more is 5 x 10 ⁸ / cm ² or less, and the yield ratio is 0.85 to 0.95.

First, the alloy composition of the hot-rolled steel sheet of the present invention will be described in detail.

Carbon (C): 0.03 to 0.1 wt%

Carbon is the most economical and effective element to strengthen the river. When the content of carbon is less than 0.03 wt%, the reaction with precipitation elements such as Ti, Nb and V is small and the precipitation strengthening effect is low. On the other hand, when the content of carbon is more than 0.1% by weight, excessive strength increase occurs, and weldability, formability and impact resistance are deteriorated. Therefore, the carbon content is preferably limited to 0.03 to 0.1% by weight.

Silicon (Si): 0.01 to 0.2 wt%

Silicon is an element added for deoxidizing molten steel and improving the strength by solid solution strengthening. In addition, it is an element effective for forming a uniform ferrite structure because it has the effect of promoting ferrite transformation during cooling after hot rolling as a ferrite stabilizing element. When the content of silicon is less than 0.01% by weight, the effect of stabilizing ferrite is small and it is difficult to make the base structure into a ferrite structure. On the other hand, if the content of silicon exceeds 0.2% by weight, a red color scale due to silicon is formed on the surface of the steel sheet during hot rolling, thereby deteriorating the surface quality of the steel sheet and deteriorating the weldability. Therefore, the content of silicon is preferably limited to 0.01 to 0.2% by weight.

Manganese (Mn): 0.8-2.0 wt%

Manganese, like silicon, is an effective element in strengthening the steel. In order to exhibit such effects, the content of manganese in the present invention is preferably 0.8 wt% or more. On the other hand, when the content of manganese exceeds 2.0 wt%, the ferrite transformation is excessively delayed and it is difficult to secure a proper fraction of ferrite as a matrix. In addition, there is a problem in that the segregation portion is greatly developed at the center of thickness during slab casting in the casting process, thereby deteriorating endurance characteristics and impact resistance characteristics of the final product. Therefore, the content of manganese is preferably limited to 0.8 to 2.0% by weight.

Aluminum (Sol.Al): 0.03 to 0.1% by weight,

Aluminum is a component mainly added for deoxidation, and as an element stabilizing ferrite, there is an effect of assisting the formation of a ferrite phase in steel during cooling after hot rolling. In order to exhibit such effects in the present invention, it is preferable that the aluminum content is 0.03 wt% or more. On the other hand, if the content of aluminum exceeds 0.1 wt%, defects are likely to occur in the slab during continuous casting, and the surface quality is deteriorated due to occurrence of surface defects after hot rolling. Therefore, the content of aluminum is preferably limited to 0.03 to 0.1% by weight.

Chromium (Cr): 0.005 to 0.3% by weight,

Chromium is an effective element for strengthening the steel, and it plays a role in delaying the transformation of bainite during cooling after welding to help formation of martensite. In order to exhibit such effects in the present invention, it is preferable that the content of chromium is 0.005 wt% or more. On the other hand, when the content of chromium exceeds 0.3% by weight, the ferrite transformation is excessively delayed to form more than necessary martensite, and the elongation is reduced. Therefore, the content of chromium is preferably limited to 0.005 to 0.3% by weight.

Molybdenum (Mo): 0.005 to 0.01% by weight,

Molybdenum is an effective element for strengthening the steel, and when added together with titanium, niobium and vanadium forms a fine complex precipitate and contributes greatly to precipitation strengthening. In order to exhibit such effects in the present invention, the content of the molybdenum is preferably 0.005% by weight or more. On the other hand, when the content of the molybdenum exceeds 0.01% by weight, the ductility decreases due to an increase in the incombustibility, and the weldability is increased, which is economically disadvantageous. Therefore, the content of the molybdenum is preferably limited to 0.01 to 0.2% by weight.

Phosphorus (P): 0.005 to 0.05 wt%

Phosphorus, like carbon, has a very strong solubility effect and is a useful element that can achieve high strength in small amounts. If the content of phosphorus is less than 0.005% by weight, however, it is insufficient to obtain the desired strength. If the content of phosphorus exceeds 0.05% by weight, softness and impact resistance may be deteriorated due to generation of band structure by micro segregation. Therefore, the content of phosphorus is preferably limited to 0.005 to 0.05% by weight.

Sulfur (S): 0.0005 to 0.05 wt%

Sulfur is an impurity present in the steel and is preferably controlled as low as possible. Theoretically, it is advantageous to control the sulfur content to 0%, but it is inevitably contained inevitably in the manufacturing process. If the content of sulfur exceeds 0.05% by weight, it forms a nonmetallic inclusion by binding with Mn or the like, thereby increasing the toughness of the steel There is a problem to drop. Therefore, the content of sulfur is preferably limited to 0.0005 to 0.05% by weight.

Nitrogen (N): 0.001 to 0.01 wt%

Nitrogen is a typical solid solution strengthening element together with carbon and forms coarse precipitates together with titanium, aluminum and the like. Generally, the solid solution strengthening effect of nitrogen is superior to carbon, but the toughness is significantly decreased as the amount of nitrogen in the steel is increased. If the content of nitrogen is less than 0.001% by weight, it takes a long time to perform the steelmaking and productivity is deteriorated. When the content of nitrogen exceeds 0.01% by weight, formation of coarse nitride is easy, . Therefore, the content of nitrogen is preferably limited to 0.001 to 0.01% by weight.

On the other hand, it is preferable to include at least one selected from the group consisting of Ti, Nb and V. In this case, the sum of the contents of Ti, Nb and V is preferably limited to 0.001 to 0.25% in total.

Ti exists as TiN in the steel and has an effect of inhibiting the growth of crystal grains during the heating process for hot rolling. In addition, Ti remaining in reaction with nitrogen is dissolved in the steel to be bonded with carbon to form TiC precipitate, which is an element useful for improving the strength and yield ratio of steel.

Nb and V form carbides in the steel, which are effective for fine grain refinement and form fine precipitates, which greatly improves the strength, toughness and yield ratio of steel. In addition, it also stabilizes the elements such as C and N that increase local variations in microstructure and physical properties due to segregation in the steel, thereby improving the impact resistance.

The remainder includes Fe and unavoidable impurities. Addition of an effective component other than the above-mentioned composition is not excluded.

Hereinafter, the microstructure and precipitates of the hot-rolled steel sheet according to the present invention will be described in detail.

The hot-rolled steel sheet according to the present invention satisfies the above-described composition conditions and has a microstructure in which the ferrite has a cross-sectional area ratio of 90% or more and a second phase containing at least one selected from the group consisting of pearlite and bainite, % Or less, and by securing the microstructure as described above, sufficient ductility can be ensured. If the cross-sectional area ratio of the second phase is more than 10%, bainite and coarse carbonitride are formed around the ferrite grain boundaries to deteriorate the ductility, impact resistance and fatigue characteristics of the steel.

In the hot-rolled steel sheet according to the present invention, the Ti, Nb and V alone or composite carbonitrides having a diameter of 100 nm or more is preferably 5 x 10 ⁸ / cm 2 or less. By reducing the coarse carbonitride, local concentration of stress on the external impact is suppressed, and the impact resistance of the hot-rolled steel sheet is excellent.

On the other hand, the hot-rolled steel sheet of the present invention has a yield ratio of 0.85 to 0.95 in addition to the alloy composition. When the yield ratio is less than 0.85, the endothelial property is disadvantageously reduced. On the other hand, when the yield ratio is more than 0.95, there is a problem that the shape fixation is weakened during part processing.

On the other hand, the hot-rolled steel sheet according to the present invention as described above can secure an impact energy of 60 J or more at -20 캜 and can be suitably applied to automobile chassis parts and structural members.

On the other hand, the hot-rolled steel sheet according to the present invention has a hot-dip galvanized layer formed on its surface, and can be used as a hot-dip galvanized steel sheet.

Hereinafter, a method of manufacturing a high strength and high strength steel sheet having excellent impact resistance characteristics, which is another aspect of the present invention, will be described in detail.

A method of manufacturing a high strength and high strength hot rolled steel sheet excellent in impact resistance, which is another aspect of the present invention,

0.001 to 0.3% of Cr, 0.005 to 0.01% of Mo, 0.005 to 0.01% of Cr, 0.01 to 0.2% of Si, 0.8 to 2.0% of Mn, 0.03 to 0.1% 0.001 to 0.05% of S, 0.0005 to 0.05% of N and 0.001 to 0.01% of N and 0.01 to 0.25% in total of at least one selected from the group consisting of Ti, Nb and V and the balance Fe and unavoidable impurities Continuously casting molten steel to obtain a slab;

Cooling the slab so as to satisfy the condition of the following expression (1);

Reheating the cooled slab to 1200 to 1300 占 폚;

Hot-rolling the reheated slab to obtain a hot-rolled steel sheet;

Cooling the hot-rolled steel sheet at a cooling rate of from 10 to 100 DEG C / s from 550 to 750 DEG C; And

Winding the cooled hot-rolled steel sheet at 550 to 750 ° C,

The step of obtaining the hot-rolled steel sheet includes a step of subjecting to hot-rolling the finish to satisfy the following condition (2)

Wherein the rolled steel sheet has a yield ratio of 0.85 to 0.95.

 [Relation 1]

Cr (° C./sec) ≥45.5-56.1 [C] +2.1 [Si] -19.2 [Mn] -8.9 [Cr] + 8.0 [Sol.Al]

(Wherein CR represents the cooling rate, and [C], [Si], [Mn], [Cr], [Sol.Al] and [Mo]

 [Relation 2]

(FET-FDT) (占 폚)? 166 - 456 [C] - 27.9 [Mn] + 4.39 [Si] - 28.5 [Mo] - 28.2 [Ti]

(C), [Mn], [Si], [Mo], [Ti] and [Nb] of the element are the same as those of the element Content (% by weight)

Steps to Obtain Slab

The molten steel satisfying the above composition is continuously cast to obtain a slab, and the slab is cooled to satisfy the following relational expression (1). This is to prevent formation of coarse carbides and nitrides by avoiding ferrite transformation occurring during cooling and suppressing the segregation or diffusion of the steel components generated during the phase transformation. Furthermore, the slab during re-heating in the heating as will allow the re-employment of the alloying elements to obtain a relatively low reheating rapidly occurs at a temperature to uniform and fine austenite phase, and after hot rolling, coarse precipitates diameter than 100nm in the hot-rolled steel sheet 5x10 ⁸ Cm < 2 > or less, and the impact resistance characteristics are improved.

[Relation 1]

Cr (° C./sec) ≥45.5-56.1 [C] +2.1 [Si] -19.2 [Mn] -8.9 [Cr] + 8.0 [Sol.Al]

(Wherein CR represents the cooling rate, and [C], [Si], [Mn], [Cr], [Sol.Al] and [Mo]

Steps to reheat

The cooled slab is reheated at a temperature of 1200 to 1300 ° C. If the reheating temperature is less than 1200 ° C, the precipitates are not sufficiently reused, and precipitates such as NbC and TiC are reduced in the process after the hot rolling. If the reheating temperature is higher than 1300 ° C, the strength is lowered due to abnormal grain growth of the austenite grains , And the reheating temperature is preferably limited to 1200 to 1300 ° C.

Step of obtaining hot-rolled steel sheet

The reheated slab is hot-rolled to satisfy the following relational expression 2 to obtain a hot-rolled steel sheet. This is because it is possible to minimize the precipitation strengthening effect after hot rolling by minimizing precipitation of precipitates due to dynamic deformation of the steel sheet during hot rolling and to maximize the strain energy in the steel sheet during hot rolling to obtain fine and uniform ferrite So that the crystal grains are formed.

[Relation 2]

(FET-FDT) (占 폚)? 166 - 456 [C] - 27.9 [Mn] + 4.39 [Si] - 28.5 [Mo] - 28.2 [Ti]

(C), [Mn], [Si], [Mo], [Ti] and [Nb] of the element are the same as those of the element Content (% by weight)

Cooling step

The hot-rolled steel sheet is cooled to 550 to 750 ° C at a cooling rate of 10 to 100 ° C / sec. If the temperature is lower than 550 占 폚 in the cooling step, most of the microstructure in the steel has bainite, so that the microstructure of the present invention can not be secured. On the other hand, if it exceeds 750 ° C, coarse ferrite and pearlite structure are formed, and desired strength can not be obtained. When the cooling rate is less than 10 ° C / sec, coarsening of the ferrite grains occurs and the precipitates are coarsened, making it difficult to obtain the desired high strength steel. On the other hand, when the temperature exceeds 100 ° C / sec, the low temperature ferrite fraction increases, .

Winding  step

Thereafter, the cooled hot-rolled steel sheet is rolled at 550 to 750 ° C. If the coiling temperature is less than 550 캜, low-temperature ferrite and bainite structure are formed to greatly reduce ductility, and formation of precipitates is suppressed to reduce strength. On the other hand, when the coiling temperature exceeds 750 캜, coarse ferrite There is a problem that crystal grains are formed and precipitates are coarsened to decrease strength. Therefore, the winding temperature is preferably limited to 550 to 750 ° C.

On the other hand, the hot-rolled steel sheet thus wound can be manufactured as a pickling steel sheet through a step of removing the surface layer scale by oil-cooling after being air-cooled at a temperature of from room temperature to 200 deg. If the temperature exceeds 200 占 폚 at the pickling step, there is a problem that the surface layer portion of the hot-rolled steel sheet is over-saponified to degrade the surface layer roughness. Therefore, the pickling temperature is preferably limited to 200 占 폚 or lower.

After the winding or pickling, the hot-rolled steel sheet may be heated at 450 to 480 ° C and passed through a hot-dip galvanizing bath to produce a hot-dip galvanized steel sheet. If the heating temperature is lower than 450 ° C, unplating tends to occur. On the other hand, if the heating temperature is higher than 480 ° C, plating defects may occur or it may be difficult to uniformly manufacture the thickness of the plating layer. Therefore, the heating temperature is preferably limited to 450 to 480 ° C.

Hereinafter, the present invention will be described in more detail with reference to examples. However, the description of these embodiments is intended only to illustrate the practice of the present invention, but the present invention is not limited thereto.

( Example )

Table 1 below shows the composition of steel slabs having the composition of inventive steels and comparative steels according to the present invention. In Table 2, slab cooling conditions, hot rolling conditions, and coiling temperatures are shown for the steel types shown in Table 1. In Table 2, CR, FET, FDT and CT are respectively the slab cooling rate, the finish rolling start temperature in hot rolling, the finishing rolling finish temperature in hot rolling and the coiling temperature, and the average of the hot rolled plate to the coiling temperature immediately after hot rolling The cooling rate was constant at 60 to 80 ° C / sec.

Table 3 shows the mechanical properties of the inventive steel and the comparative steel, the results of the impact resistance evaluation, and the microstructure observation results. In Table 3, YS, TS, T-El and YR mean yield strength, tensile strength, fracture elongation and Yield Ratio (YR = YS / TS) Or lower yield point. The tensile test was performed on specimens taken in accordance with the JIS No. 5 standard with respect to the rolling direction of the rolled plate in the direction of 90 °. The results of the tensile tests shown in Table 3 are the values averaged after three runs. The second phase fraction of the microstructures was measured with a Lepera etchant or a Nital etchant, and then observed with an optical microscope at a magnification of 500, and analyzed with an image analyzer. Coarse Ti, Nb and V alone or complex carbonitrides were prepared by Replica method and observed by TEM (Transmission Electron Microscopy). In addition, the impact resistance of the hot-rolled steel sheet shown in Table 3 is the result obtained by testing with ASTM Standard E8m-04 standard. At this time, the impact test specimens were taken in the vertical direction in the rolling direction, and the impact energy was the smallest among the results of three tests at -20 ° C. When the impact absorption energy value is smaller than 60J, it is judged that the impact resistance characteristic is heat.

Psalter C Si Mn Cr Sol.Al P S N Ti Mo Nb V Comparative River 1 0.045 0.1 1.6 0.01 0.03 0.01 0.003 0.004 0.02 0.15 0.04 0.005 Comparative River 2 0.05 0.4 1.4 0.1 0.03 0.01 0.003 0.004 0.05 0.007 0.01 0.005 Comparative Steel 3 0.06 0.1 1.7 0.01 0.03 0.01 0.003 0.004 0.02 0.008 0.02 0.005 Comparative Steel 4 0.07 0.2 1.8 0.01 0.15 0.01 0.003 0.004 0.02 0.007 0.04 0.005 Comparative Steel 5 0.08 0.3 1.9 0.2 0.03 0.01 0.003 0.004 0.08 0.005 0.005 0.025 Comparative Steel 6 0.07 0.2 1.5 0.1 0.03 0.01 0.003 0.004 0.12 0.008 0.02 0.05 Comparative Steel 7 0.09 0.1 1.8 0.01 0.05 0.01 0.003 0.004 0.08 0.009 0.03 0.005 Comparative Steel 8 0.095 0.05 1.8 0.01 0.03 0.01 0.003 0.004 0.12 0.007 0.025 0.005 Comparative Steel 9 0.12 0.2 1.8 0.01 0.5 0.01 0.003 0.004 0.12 0.006 0.015 0.02 Inventive Steel 1 0.035 0.05 1.2 0.01 0.03 0.01 0.003 0.005 0.06 0.006 0.02 0.005 Invention river 2 0.07 0.05 0.8 0.01 0.03 0.01 0.003 0.004 0.04 0.005 0.03 0.005 Invention steel 3 0.065 0.05 1.2 0.01 0.03 0.01 0.003 0.004 0.04 0.006 0.04 0.005 Inventive Steel 4 0.08 0.05 1.2 0.01 0.03 0.01 0.003 0.003 0.055 0.007 0.045 0.005 Invention steel 5 0.07 0.15 1.7 0.01 0.03 0.01 0.003 0.004 0.14 0.006 0.025 0.005 Invention steel 6 0.07 0.1 1.5 0.2 0.03 0.01 0.003 0.003 0.09 0.009 0.03 0.005 Invention steel 7 0.075 0.05 1.7 0.01 0.03 0.01 0.003 0.004 0.1 0.007 0.01 0.06 Inventive Steel 8 0.085 0.1 1.8 0.2 0.03 0.01 0.003 0.004 0.1 0.008 0.06 0.04

Psalter CR (° C / sec) ① FET (° C) FDT (占 폚) FET-FDT (占 폚) ② CT (° C) Comparative River 1 14 8.6 986 895 91 94 566 Comparative River 2 17 15.8 992 893 99 104 610 Comparative Steel 3 5 9.6 1005 892 113 90 608 Comparative Steel 4 6 8.4 985 910 75 82 615 Comparative Steel 5 10 3.5 991 890 101 75 604 Comparative Steel 6 14 12.3 980 896 84 88 540 Comparative Steel 7 5 6.2 1000 901 99 71 614 Comparative Steel 8 8 5.7 992 898 94 68 638 Comparative Steel 9 6 8.4 998 905 93 58 545 Inventive Steel 1 22 20.6 1001 899 102 114 618 Invention river 2 28 26.3 998 905 93 109 590 Invention steel 3 20 18.9 994 909 85 100 616 Inventive Steel 4 20 18.0 996 912 84 92 611 Invention steel 5 12 9.2 991 915 76 82 625 Invention steel 6 13 11.2 1002 918 84 88 626 Invention steel 7 10 8.7 989 913 76 81 618 Inventive Steel 8 6 4.6 982 915 67 71 630 [Mo] - 8.9 [Cr] + 8.0 [Sol.Al] - 26.9 [Mo]
- = 28.9 [Si] - 28.5 [Mo] - 28.2 [Ti] - 51.1 [Nb]

Psalter YS
(MPa) TS
(MPa) T-El
(%) YR Second phase
Phase fraction (%) Number of coarse precipitates
(EA / cm ² ) Shock energy
(J) Impact resistance Comparative River 1 584 675 20 0.87 13 2.2 x ^{10 8} 95.4 O Comparative River 2 564 625 24 0.90 11 4.4 x ^{10 8} 101.8 O Comparative Steel 3 515 564 26 0.91 7 5.7 x ^{10 8} 54.3 X Comparative Steel 4 594 645 23 0.92 5 5.3x10 ⁸ 57.9 X Comparative Steel 5 690 740 22 0.93 6 6.0x10 ⁸ 48.2 X Comparative Steel 6 667 794 21 0.84 11 4.8x10 ⁸ 69.3 O Comparative Steel 7 684 732 19 0.93 6 6.4 x ^{10 8} 51.6 X Comparative Steel 8 755 767 19 0.98 5 7.3 x ^{10 8} 42.3 X Comparative Steel 9 660 788 17 0.84 9 6.7 x ^{10 8} 46.8 X Inventive Steel 1 478 548 28 0.87 6 2.5x10 ⁸ 105.2 O Invention river 2 510 553 27 0.92 5 3.7x10 ⁸ 99.4 O Invention steel 3 553 596 25 0.93 5 3.1x10 ⁸ 92.5 O Inventive Steel 4 605 653 23 0.93 5 3.0x10 ⁸ 96.1 O Invention steel 5 774 832 19 0.93 4 4.3 x ^{10 8} 73.5 O Invention steel 6 723 773 20 0.94 4 3.9x10 ⁸ 80.9 O Invention steel 7 685 768 19 0.89 6 3.3 x ^{10 8} 83.6 O Inventive Steel 8 847 912 15 0.93 4 4.8x10 ⁸ 67.6 O

The comparative steels 1 and 2 exhibited excellent impact resistance characteristics in accordance with the conditions proposed in the present invention, with the cooling rate CR of the slab and the temperature condition during the hot rolling at the finish. However, in Comparative Steel 1, the content of Mo in the steel component exceeded the range of the present invention, so that an amorphous ferrite phase and a bainite phase were formed in the microstructure due to the increase of the entrapment due to Mo, and the comparative steel 2 contained Si The content exceeded the range of the present invention, and the microstructure volume fraction of the microstructure other than ferrite exceeded 10%. The composition range of Comparative Steel 3 was in good agreement with the proposed range of the present invention, but the impact resistance was poor because the cooling rate of the slab and the temperature condition at the finish hot rolling did not match with the relational expressions 1 and 2. In Comparative Steel 4, the content of Sol.Al exceeded the range of the present invention, and the cooling rate of the slab failed to comply with the relational expression 1, so that the impact resistance characteristic was weakened. The comparative steel 5 had an impulse resistance characteristic due to the failure of the temperature condition during the final hot rolling to be in accordance with the relationship of Equation 2. The comparative steel 6 had a coiling temperature outside the range controlled by the present invention and a volume fraction of the second phase of the microstructure was 10% , And the yield ratio (YR) is out of the scope of the present invention. The comparative steel 7 failed to satisfy all of the relational expressions 1 and 2 and the impulse characteristics were disappeared. The comparative steel 8 did not satisfy all the relational expressions 2, and the impact resistance characteristic was weakened. The yield ratio (YR) . In Comparative Steel 9, the yield ratio of C and Sol.Al exceeded the range of the present invention and the yield ratio was low due to the formation of the bainite phase.

The yield ratio (YR) and the impact energy value of comparative steels 1 to 9 and inventive steels 1 to 8 are shown in Fig. The hatched area in FIG. 1 corresponds to the steel range of the present invention.

Claims (8)

0.001 to 0.3% of Cr, 0.005 to 0.01% of Mo, 0.005 to 0.01% of Cr, 0.01 to 0.2% of Si, 0.8 to 2.0% of Mn, 0.03 to 0.1% 0.001 to 0.25% in total of at least one selected from the group consisting of Ti, Nb and V, 0.001 to 0.25% of S, 0.0005 to 0.05% of N, and 0.001 to 0.01% of N and the balance Fe and unavoidable impurities ≪ / RTI &
A high strength, high strength, high strength hot rolled steel sheet excellent in impact resistance characteristics, wherein the number of Ti, Nb and V alone or composite carbonitrides having a diameter of 100 nm or more is 5 x 10 ⁸ / cm ² or less and a yield ratio is 0.85 to 0.95.
The method according to claim 1,
The microstructure of the hot-rolled steel sheet is a high-strength, high-strength, high-strength hot-rolled steel sheet in which the cross-sectional area ratio of the ferrite is 90% or more.
The method according to claim 1,
Wherein the hot-rolled steel sheet has an impact energy of 60 J or more at -20 캜.
The method according to claim 1,
Wherein the hot-rolled steel sheet further comprises a hot-dip galvanized layer.
0.001 to 0.3% of Cr, 0.005 to 0.01% of Mo, 0.005 to 0.01% of Cr, 0.01 to 0.2% of Si, 0.8 to 2.0% of Mn, 0.03 to 0.1% 0.001 to 0.05% of S, 0.0005 to 0.05% of N and 0.001 to 0.01% of N and 0.01 to 0.25% in total of at least one selected from the group consisting of Ti, Nb and V and the balance Fe and unavoidable impurities Continuously casting molten steel to obtain a slab;
Cooling the slab so as to satisfy the condition of the following expression (1);
Reheating the cooled slab to 1200 to 1300 占 폚;
Hot-rolling the reheated slab to obtain a hot-rolled steel sheet;
Cooling the hot-rolled steel sheet at a cooling rate of from 10 to 100 DEG C / s from 550 to 750 DEG C; And
Winding the cooled hot-rolled steel sheet at 550 to 750 ° C,
The step of obtaining the hot-rolled steel sheet includes a step of subjecting to hot-rolling the finish to satisfy the following condition (2)
Wherein the rolled hot-rolled steel sheet has a yield ratio of 0.85 to 0.95.
[Relation 1]
Cr (° C./sec) ≥45.5-56.1 [C] +2.1 [Si] -19.2 [Mn] -8.9 [Cr] + 8.0 [Sol.Al]
(Wherein CR represents the cooling rate, and [C], [Si], [Mn], [Cr], [Sol.Al] and [Mo]
[Relation 2]
(FET-FDT) (占 폚)? = 166 - 456 [C] - 27.9 [Mn] + 4.39 [Si] - 28.5 [Mo] - 28.2 [Ti]
(C), [Mn], [Si], [Mo], [Ti] and [Nb] of the element are the same as those of the element Content (% by weight)
6. The method of claim 5,
Picking up the hot-rolled steel sheet after the winding step; And
Further comprising the step of raising the pickled hot-rolled steel sheet, and further comprising the step of raising the pickled hot-rolled steel sheet.
6. The method of claim 5,
Picking up the hot-rolled steel sheet after the winding step; And
Further comprising a step of immersing the hot-rolled steel sheet subjected to the pickling treatment in a hot-dip galvanizing bath at 450 to 480 캜 and hot-dip galvanizing the hot-rolled steel sheet.
8. The method according to claim 6 or 7,
Wherein the pickling treatment is carried out at a temperature of 200 DEG C or less.

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